The popularity of video distribution over the Internet has brought with it a need to increase the recording capacity of the servers that serve as information sources for such distribution. To this end, an increase in data distribution speed and recording capacity has been accomplished in server applications by employing a RAID structure. The number of hard disk drives (hereinafter referred to as HDDs) has to be increased to use a RAID structure, so the general practice has been to install a plurality of HDDs in a tower housing. However, since this is limited to the amount of installation space that is available, it is necessary at the same time to increase the recording capacity of each individual HDD. It is therefore important to increase the number of disks installed in a HDD.
Also, it is becoming increasingly common for individual users to record to a HDD rather than a conventional VCR. Here again, users want to be able to record distributed video to a HDD for as long a time as possible, so there is a strong demand for greater recording capacity in HDDs.
In the past, one method for increasing the number of disks that could be installed in a HDD was to fix the top end of a motor shaft with a screw to the cover constituting the HDD housing, and use the rigidity of the shaft to suppress vibration of the housing.
A structure here in which a shaft is fixed at both ends is also called a “tied” structure, and is often used in structures that involve ball bearings.
Meanwhile, there has been a sharp increase in the planar recording density of HDDs, and particularly the track density, and it has been difficult to keep NRRO (Non-Repetitive Run Out) to 100 nm or less with conventional ball bearings, and from the standpoint of noise and impact resistance as well, ball bearings are gradually being replaced by hydrodynamic bearings.
However, a hydrodynamic bearing needs to be designed so as to suppress leakage or evaporation of the lubricant (lubricating oil). In particular, with a hydrodynamic bearing having a tied structure, in which both ends are open, more structural modification is necessary than with an untied structure, which has a pouch-like oil reservoir. More specifically, there is greater importance in the seal structure at openings where the liquid surface is exposed to the air.
There have been many proposals for the seal structures of hydrodynamic bearings, and particularly the constitution near the outer ends thereof (see, for example, Patent Documents 1 to 3).
In all of Patent Documents 1 to 3, the constitution is a spindle motor for a HDD, in which a shaft is fixed to a base or chassis. A sleeve is rotatably provided around the shaft. Radial dynamic pressure generation grooves are formed in a herringbone pattern at two vertical levels in the axial direction around the inner peripheral cylindrical face of the sleeve, and the sleeve pairs with the shaft to constitute a radial bearing.
Also, thrust flanges that are fixed to the shaft are provided at above and below positions in the axial direction of the radial bearing. The thrust flanges are disposed across from the ends of the axial end faces of the sleeve. Thrust dynamic pressure generation grooves are formed in a spiral or herringbone pattern on the sleeve-side end faces of the thrust flanges, and constitute thrust bearings between the thrust flanges and the sleeve.
A lubricant continuously fills the space between the radial bearing and the thrust bearings. Openings those expose the liquid surfaces of the lubricant to the atmosphere are provided at vertical positions in the axial direction of two thrust bearings, and seal structures are formed in the openings. These seal structures prevent the lubricant from leaking out while the hydrodynamic bearing is stopped, rotating, in shipment, or being installed in an instrument.
Also, a thrust vertical communication hole is provided at a radial position of the sleeve across from the outer peripheral edges of the thrust flanges in the axial direction, so as to communicate with the vertical thrust bearings in the axial direction. This thrust vertical communication hole is also filled with lubricant. The effect of this thrust vertical communication hole is that when there is variance in the assembly precision or machining precision of the thrust bearings or the radial bearing, even if the pumping force of the each bearing (a force that tries to push the lubricant in one direction) should be non-uniform, the pressure will be substantially the same as atmospheric pressure because the thrust vertical communication hole is near the opening of the bearing. This makes it possible to equalize the pressure differential between the bearings, which means that the lubricant will be less likely to leak during rotation.
Further, female threads are formed at one end of the shaft, and the HDD cover is fixed to the shaft with female threads. With a constitution such as this, the HDD cover and the lower chassis are securely fixed via the shaft. As a result, the rigidity of the overall housing is greatly increased, resistance to vibration and shock is improved, and there is less chassis vibration, which is caused by motor vibration that occurs during motor rotation. Accordingly, there is less transmission of harmful vibration to a voice coil motor (VCM) to which the recording and reproduction head is mounted, which affords higher-density recording. Furthermore, even if there is an increase in the number of disks, which account for the majority of the rotor weight, it will be possible to reduce the rocking mode frequency component attributable to the bearings. As a result, many more disks can be accommodated than with a single-ended motor in which the motor shaft is supported at just one end, and the overall recording capacity of the HDD can also be increased by several multiples.
Patent Documents 1 to 3 disclose a peripheral wall that is either fixed or integrally machined in the sleeve or hub. This will be described through reference to FIG. 18.
A peripheral wall 302 is integrally formed on the axial upper side (the upper and lower directions in the drawing are referred to as the axial upper side and axial lower side) of a sleeve 300. As shown in FIG. 18, the peripheral wall 302 is provided in order to facilitate the centering of a clamping member 306 for clamping a disk 304 during the fixing of the clamping member 306 to the outer periphery of a hub 308 fixed to the outer peripheral side of the sleeve 300. With the structure disclosed in Patent Document 2, the distal end of the shaft is formed lower than the distal end of the peripheral wall. This is because the thickness of the HDD is determined by de facto standard, so the female threads must not protrude past the HDD cover when the HDD cover is fixed with female threads.
The peripheral wall is constituted so as to cover the vertical (axial direction) seal structure from the outer peripheral side. The peripheral wall thus serves as a barrier that prevents any jig or a worker from accidentally coming into contact with the seal structure in the course of assembly.
With the constitution given in Patent Documents 1 to 3, the hydrodynamic bearing is open at both ends, which makes the lubricant sealing performance at both ends especially important. In particular, the lubricant must not leak from either open end even if the hydrodynamic bearing device should be subjected to turbulent vibration, impact, or the like, or if the orientation of the device should change, whether in the completed state of the hydrodynamic bearing device, the completed state of the motor equipped with the hydrodynamic bearing device, or the completed state of the drive equipped with that motor. To that end, it is necessary to achieve good precision in the shape of the seal structure, the structure of the bearings, the dimensional tolerances, and so forth, and to equalize the pressure within the bearing at the top and bottom with the above-mentioned thrust vertical communication hole.
Up until the cover is in place and the HDD is complete, there is generally nothing that serves as a protective cover from the top side (the cover side) of the hydrodynamic bearing device. Consequently, there is the possibility that this side will come into contact with a jig or a finger through the opening on the cover side. This generally occurs, for example, when a person's finger touches the hydrodynamic bearing device in the course of visually inspecting the motor or the hydrodynamic bearing device.
For instance, as shown in FIG. 19, if a person's finger Fg1 touches and applies even just a little more force (see the finger Fg2) so as to block off the peripheral wall 302 of the sleeve 300, the air near the opening 310 will be compressed and apply pressure to the lubricant liquid surface.
Usually, the surface level of the lubricant in a hydrodynamic bearing device is located in the middle (axial direction) of the tapered opening 310 (see L1 in the drawing). In a state in which force is applied to a finger blocking off the peripheral wall 302, the volume of the space located to the inside (radial direction) of the peripheral wall 302 is reduced by the volume indicated by S1, and the liquid level is lowered at the opening 310 by an amount corresponding to this reduction (see L2). As a result, there is the possibility that the liquid level at the other tapered opening in the axial direction will move closer to the open end side through the thrust vertical communication hole 312, eventually causing the lubricant to overflow from the opening 314. Leakage of the lubricant from the other opening 314 is particularly apt to occur when the peripheral wall 302 is blocked off very suddenly. This is because the concussive application of pressure to the lubricant surface makes it impossible for the surface tension alone at the tapered opening 314 to withstand both the pressure that is transmitted through the lubricant and the lubricant weight.
It can be confirmed by visual inspection whether the lubricant has leaked before the hydrodynamic bearing device is installed in the motor. In this case, it is a relatively simple matter to remove from the assembly line any products that do not pass inspection because of lubricant leakage. However, once the hydrodynamic bearing device has been installed in the motor, or when that motor has been installed in a HDD, it is difficult to tell whether the lubricant has leaked.
When the peripheral wall 302 (see FIG. 19) is blocked off more gradually, there is no concussive pressure fluctuation, so the lubricant can be retained in the tapered opening 314. Nevertheless, even if the lubricant does not leak from the opening 314, if the lubricant is pushed down at the opening 310, this allows air bubbles to get into the thrust bearing 316 on the axial upper side, which communicates with the opening 310. When this happens, if the hydrodynamic bearing device starts rotating while these bubbles are admixed, there may be lubricant breakdown in the thrust bearing or radial bearing, which can lead to motor lock within a short time. The admixture of bubbles in the thrust bearing or radial bearing is difficult to ascertain by visual inspection, so the infiltration of these bubbles must be prevented.
Patent Document 1: Japanese Laid-Open Patent Application 2001-355631
Patent Document 2: Japanese Laid-Open Patent Application 2002-311131
Patent Document 3: Japanese Laid-Open Patent Application 2005-16672